566 Forty-fifth Report on the State Museum. 



Type, Acrothele coriacea, Linnarsson. 



Distribution. Cambrian — -Lower Silurian (New Brunswick, 

 Utah, Ireland, Bohemia, Sweden). 



Acrotreta, Kutorga. 1888. 

 (Plate 3, Figs 28-30.) 



Pedicle-valve subconical, the posterior slope being flattened, 

 having a triangular form and resembling a cardinal area. This 

 slope is divided medially by a groove, widening downward ; at the 

 apex of the valve is a circular foramen. On the interior this 

 foramen penetrates a mammiform callosity. 



Brachial valve flat or depressed convex ; apex marginal ; on 

 the interior is a stout median ridge flanked by a large anterior 

 and a smaller posterior pair of muscular impressions. 



Exterior concentrically striated. Shell substance corneous. 



Type, Acrotreta subconica, Kutorga. 



Distribution, Cambrian — (?) Lower Silurian. 



Conotreta, Walcott. 1888. 



(Plate 4, figs. 1-3.) 



Synonym; Geinitzia, 'Hall. 1889. 



Pedicle-valve conical, its height being greater than its length ; 

 apex with evidence of the external opening of the sipho. From 

 the apex, a shallow furrow extends to the posterior margin, 

 increasing in width downward. In young shells the posterior 

 wall of the shell conforms to the curvature of the rest of the sur- 

 face, interrupted only by the longitudinal depression, but, with 

 increase in size, this area becomes distinctly flattened as in 

 Acrotreta. Surface covered with sharp concentric stria? which 

 make a slight upward curve as they cross the foramina! groove. 



On the interior a strong apical callosity surrounds the probable 

 position of the foramen. This is somewhat produced anteriorly 

 into a short, sharp ridge, on either side of which lie two other 

 ridges, with evidence of a third on the lateral slopes. 



Type, Conotreta Rust/', Walcott. (Lower Silurian.) 



The single species described occurs in the Trenton limestone of 

 New York and the Utica slate of Kentucky. 



Discinopsis, Matthew. 18''2. 

 (Plate 3, Figs. 20, 21 .) 

 Shell subcircular in outline. Surface depressed conical, apices 

 excentric, not marginal. Pedicle-valve with the apex truncated 



lis 



